Does a 350 Chevrolet engine fit into 1986 S-10 Blazer?

No, a 350 Chevrolet small-block engine (like the ones commonly found in full-size Chevrolets and other GM vehicles) will not fit into a 1986 S-10 Blazer without significant modifications. The S-10's engine bay is considerably smaller than what's needed to accommodate a 350. You'd have major clearance issues with the firewall, hood, and possibly the frame. While some people undertake such ambitious projects, it requires extensive fabrication and custom work, making it a very complex and costly undertaking.